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| Flow of Energy and Information |
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| Living cells constantly exchange energy and materials with their surrounding environment. The functioning of living cells depends upon the flow of energy and flow of information. |
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| Every organism has a tendency to increase its entropy, which is very much in accordance with the second law of thermodynamics. In order to maintain its organisation at the molecular, cellular and organismic levels, every organism is forced to obtain energy from its surroundings, so that the state of entropy can be reduced as much as possible. Energy for this purpose may be derived from the photons of radiant energy or in the form of bond energy that is available from the oxidation of food molecules. The flow of energy involves three processes in the living system, namely photosynthesis, cellular respiration and breakdown of ATP. Photosynthesis is responsible for transformation of solar energy into chemical energy. Cellular oxidation is responsible for conversion of chemical energy into the biologically useful energy in the form of energy rich phosphate bonds in adenosine triphosphate (ATP). The breakdown of ATP results in the release of chemical energy in the phosphate bonds to be utilised in the cells to carry on its activities. |
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| fig. 12.5 - Flow of Energy |
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| The flow of information in the living cells involves two mechanisms. One of them is the flow of genetic information contained in the DNA. This information, which is responsible for all cellular activities, is present in the form of a specific sequence of nucleotides and is called as the genetic code. This information gets transcribed into RNA in the form of triplet combinations of nucleotides, called codons. The specific sequence of these codons in a type of RNA called messenger RNA (mRNA), decides the sequence in which the amino acids are to be arranged to form the required protein. Thus, the genetic information in mRNA is translated into the specific protein molecules. The other form of information is the extrinsic information. Certain chemical substances like hormones carry information from outside the cell. Such substances enter the cell and influence its activities, by binding with substances called receptors within the cell. Some of these hormones cannot enter the cell due to their large molecular size. Hence, they act at the cell surface. They bind with receptors found at the cell surface and exert their influences. Such hormones bring about significant changes in the properties of cell membranes and activities of the cell. |
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| fig. 12.6 - Flow of Information |
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| fig. 12.7 - Flow of Energy and Information in a Cell |
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